1. Field of the Invention
The present invention relates to a sheet supplying apparatus used with an image forming apparatus such as a printer, a copying machine and the like, and more particularly, it relates to a sheet supplying apparatus in which stacked sheets are separated and supplied one by one by absorbing the sheet.
2. Related Background Art
Conventionally, as one of sheet supplying apparatuses used with an image forming apparatus such as a printer, a copying machine and the like, there has been proposed a sheet supplying apparatus of air absorption type in which sheets stacked on a sheet stacking means are separated and supplied one by one by absorbing the sheet (for example, see Japanese Patent Application Laid-open No. 61-23050). Such a sheet supplying apparatus comprises a sheet absorbing means for absorbing a sheet from a sheet stack rested on a sheet stacking means by utilizing an absorbing force generated by an absorbing force generating means, and a shift means for shifting the sheet absorbing means to direct the sheet absorbed by the sheet absorbing means to a sheet supply means disposed downstream of the sheet stacking means in a sheet supplying direction, and the separated sheet is supplied to a recording portion by the sheet supply means.
The absorbing force generating means is generally constituted by a relatively large negative pressure generating pump having an exclusive motor. After the sheet is absorbed by the sheet absorbing means connected to the negative pressure generating pump and the sheet is transferred to the sheet supply means, an absorbing force is not needed until a next sheet is to be absorbed. Thus, since the absorbing force of the sheet absorbing means affects a bad influence upon the transferring of the sheet to the sheet supplying means, it is controlled such that the absorbing force acting on the sleet absorbing means is reduced to substantially zero by communicating the absorbing force to the atmosphere in an air path between the negative pressure generating pump and the sheet absorbing means. Further, since the negative force generated by the absorbing force generating means or absorbing capacity is influenced by permeability of the sheet to be absorbed, for example, when a sheet such as a plain sheet having relatively great permeability is used, the negative pressure generated by the negative pressure generating pump is set so that a desired absorbing force can be obtained in accordance with the permeability of the sheet.
However, in the above-mentioned conventional technique, since the absorbing force generating means is constituted by the relatively large negative pressure generating pump having the exclusive motor, the entire apparatus is made bulky not to save space, and, since a plurality of independent drive sources are provided, the number of parts is increased to make the apparatus expensive. Further, since the plurality of independent drive sources must be controlled in a synchronous manner, the construction of the control means for bringing the absorbing force of the sheet absorbing means to substantially zero (after the sheet is absorbed by the sheet absorbing means and then is transferred to the downstream sheet supply means) becomes complicated. In addition, when the negative pressure generated by the negative pressure generating pump or the absorbing capacity is set in dependence upon the sheet having relatively great permeability, if a sheet such as a resin film sheet having relatively small permeability is used, the absorbing force of the sheet absorbing means excessively acts on the sheet, so that sheet is deformed and/or excessive load is added to the sheet absorbing means and the negative pressure generating pump to damage the latter and to spend useless energy.